<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    const s3 = new Set(["a", "a", "b", "b"]);
    console.log(s3.size); // 2
    const ary = [...s3];
    console.log(ary);     // (2) ["a", "b"]
  </script>
</body>
</html>